High pressure piezoresistive transducer suitable for use in hostile environments

ABSTRACT

A pressure transducer including: a silicon substrate including: a first surface adapted for receiving a pressure applied thereto, an oppositely disposed second surface, and a flexing portion adapted to deflect when pressure is applied to the first surface; at least a first sensor formed on the second surface and adjacent to a center of the flexing portion, and adapted to measure the pressure applied to the first surface; at least a second gauge sensor formed on the second surface and adjacent to a periphery of the flexing portion, and adapted to measure the pressure applied to the first surface; a glass substrate secured to the second surface of the silicon wafer.

FIELD OF INVENTION

The present invention relates to pressure transducers and moreparticularly to an improved high pressure piezoresistive transducerwhich is suitable for use in hostile environments and a novel,advantageous method for making the same.

BACKGROUND OF INVENTION

Kulite Semiconductor Products, Inc., the assignee herein, has previouslymade and patented a method for fabricating high pressure piezoresistivetransducers using both longitudinal and transverse piezoresistivecoefficients U.S. Pat. No. 5,702,619, entitled “Method of Fabricating aHigh-Pressure Piezoresistive Transducer”, filed Sep. 30, 1996, andassigned to the assignee herein, the entire disclosure of which ishereby incorporated by reference. Therein, a basic sensor is formed froma piece of single crystal silicon to which sensors are dielectricallybonded on one surface and the other surface of the silicon is bonded toa glass support member. In those structures the piezoresistive elementswere formed on the surface of the transducer that is directly exposed tothe pressure media. Additionally, electrical contacts and lead wires arealso exposed to the media.

This structure is undesirable in some situations, where exposure of thepiezoresistive elements, electrical contacts and lead wires to the mediashortens the life expectancy of the pressure transducer. Accordingly, itis an object of the present invention to provide a high pressuretransducer less sensitive to the media.

SUMMARY OF INVENTION

A pressure transducer including: a silicon substrate including: a firstsurface adapted for receiving a pressure applied thereto, an oppositelydisposed second surface, and a flexing portion adapted to deflect whenpressure is applied to the first surface; at least a first sensor formedon the second surface and adjacent to a center of the flexing portion,and adapted to measure the pressure applied to the first surface; atleast a second gauge sensor formed on the second surface and adjacent toa periphery of the flexing portion, and adapted to measure the pressureapplied to the first surface; a glass substrate secured to the secondsurface of the silicon wafer.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a diagram of the approximate shape of a diaphragmwith the resistor placement according to the present invention.

FIG. 2 illustrates a top view of a glass support to which the diaphragmof FIG. 1 is mounted according to the present invention.

FIG. 3 illustrates a cross-section A—A of FIG. 2.

FIG. 4 illustrates the mounting of the diaphragm/support assembly to aheader according to the present invention.

FIG. 5 illustrates a cross-section of a diaphragm/support and headeraccording to the present invention.

FIG. 6 illustrates a perspective view of gauge placement according tothe present invention.

FIG. 7 illustrates a side view of a sensor assembly as described in U.S.Pat No. 5,614,678.

FIG. 8 illustrates a stress diagram for the sensor of FIG. 7.

FIG. 9 illustrates a sensor assembly according to the present invention.

FIG. 10 illustrates a stress diagram for the sensor of FIG. 9.

DETAILED DESCRIPTION OF INVENTION

Referring now to the numerous figures, wherein like references refer tolike elements of the invention, FIG. 1 illustrates a diagram of theapproximate shape of a diaphragm with the resistor placement accordingto the present invention.

According to the present invention, piezoresistive elements are placedon a side of the silicon structure 10 isolated, or away, or oppositefrom a media. Preferably, two elements or gauges, 15, 20 are locatednear a center of flexing portion 25 of the silicon member 10, while twoadditional members or gauges 30, 35 are located just inside the flexingportion 25 area.

Contact areas 40 on the silicon structure 10 are sealed to a glasssupport structure 45 in a non-flexing area (complement of flexing area25). Referring now also to FIGS. 2 and 3, holes 50 are provided in theglass support structure 45 to access the various contact areas 40 of thesilicon structure 10 associated with sensors 15, 20, 30 and 35.Additionally, a small depression 55 to allow the flexing area portion 25of the silicon structure 10 to deflect is provided. The sensor network(sensors 15, 20, 30 and 35) and contact areas 40 are preferablydielectrically isolated from the silicon structure 10 in the same manneras U.S. patent application Ser. No. 09/047,548, entitled “CompensatedOil-Filled Pressure Transducers” filed Mar. 25, 1998, the entiredisclosure of which is also incorporated by reference hereinto,including the seal of the glass to a rim structure and to the contactareas 40.

Referring now also to FIGS. 4 and 5, the apertures, or holes, 50 in theglass structure 45 are preferably partially filled with a metallic frit60 (and or an epoxy metal frit for example) and small copper balls 65are inserted in the back areas of the apertures 50 (also see FIG. 9).The sensor-glass support structure (collectively 10 and 45) is thenmounted to either a polymide structure 70 or ceramic structure withplated through holes 75 into which the exposed portions of the copperballs 65 will seat.

Contact can be made between the balls 65 and plated through holes 75with a solder or braze. If a polymide structure 70 is used, the sensorstructure can be secured with an epoxy or like material, while if aceramic structure 70 is used the mounting may be made using a glass typefrit. However, both mounting surfaces contain lead outs, ormetallizations 90 to a series of holes 80, sized in such a way toconform to the position of pins 95 on a header 85 preferably securedutilizing tapered glass 100.

The composite structure (10, 45 and 70) is then mounted on the header 85allowing the interconnects and the composite structure (10, 45 and 70)to be electrically connected to the pins of the header 85. When pressureis applied from the side of the silicon not containing the sensornetwork, i.e. opposite thereof, the central portion of the siliconstructure 10 deflects giving rise to a tensile surface strain in thecenter of the flexing member 25, while the exterior portions of theflexing member 25 will be put in compression.

Referring now also to FIG. 6, methods of finite analysis were used toelucidate the various stresses within the plane of the silicon structure10 (directions 105 and 110) and normal to it (direction 115). Thisanalysis shows that the region of compressive surface stress in theflexing portion 25 where the sensor may be placed is very narrow. Thisis because the compressive normal stress in the center of the flexingregion 25 is zero, but rises to its largest value at the outer edge ofthe flexing member 25, and because of the negative sign of thetransverse gage factor in the <110> direction is negative. If the outergauge is in this region, the change in resistance will be positive[(−1)×(−1)] and there will be no output.

In general, each gauge sees three different stresses: a longitudinalstress in the plane of the diaphragm (direction 110), a transversestress in the plane of the diaphragm (direction 105), and a transversestress perpendicular to the diaphragm (direction 115). These stressesserve to change the resistivity of the gauge through piezoresistiveeffects. In general this change in resistivity can be broken down into achange for each separate stress, namely: $\begin{matrix}{\frac{\Delta \quad R}{R} = {{\sigma_{x}\pi_{x}} + {\sigma_{y}\pi_{y}} + {\sigma_{z}\pi_{z}}}} & (1)\end{matrix}$

where σ is the stress is one of the three directions and Π is thepiezoresistive coefficient in that same direction.

By appropriate choice of crystallographic orientation, one skilled inthe art can ensure the coefficient in the longitudinal in plane (110)and transverse out of plane (115) are equal in magnitude and opposite insign, while the coefficient for the transverse in plane (105) is veryclose to 0. This leads for a final result for the change in resistanceto be: $\begin{matrix}{\frac{\Delta \quad R}{R} = {{\sigma_{long}\left( \frac{\pi_{44}}{2} \right)} - {\sigma_{tran}\left( \frac{\pi_{44}}{2} \right)}}} & (2)\end{matrix}$

By finite element analysis one can compute the transverse andlongitudinal stresses that the gauges see and therefore choose thelocations which yield the maximum change is resistance for a given loadcondition.

Referring now also to FIGS. 7-10, therein is illustrated a not to scaledrawing and a graph of the relevant stresses for both a conventionalhigh pressure sensor (FIGS. 7-8) and the new leadless one (FIGS. 9-10).FIGS. 7 and 9 are for reference only and should be used to clarify FIGS.8 and 10. FIGS. 8 and 10 illustrate the transverse and longitudinalstresses in the appropriate part of the diaphragm. FIGS. 8 and 10 alsohave marked the approximate locations for the placement of the gauges.

Referring first to FIG. 7, therein is illustrated a conventionalpressure transducer including supports 120, diaphragm 125 and gauges130.

It can be seen from FIGS. 8 and 10 that for each sensor there are twogauges which will see a negative change in resistance and two which willreceive a positive change in resistance. By combining these four gaugesin a wheatstone bridge as set forth in U.S. Pat. No. 3,654,579, entitled“Electromechanical Transducers and Housings” filed May 11, 1970, isassigned to the assignee hereof, also herein incorporated by reference,one can to achieve the desired change in voltage.

This new structure has a number of unanticipated advantages. Theposition of both the inner and outer gages was only learned bycomputation using finite element analysis and would be different foreach geometry of the sensor but the large difference in surface stressdistributed from the top to the bottom surface of the silicon was notanticipated. However, the use of the finite analysis still makespossible the fabrication of a miniature sensor.

By putting the sensing network on the side of the silicon away from themedia and using glass support structures with access holes to reach thecontacts, it makes possible the construction of a “leadless” structurewithout fine gold wires and ball bonds as is illustrated in pending U.S.patent application Ser. No. 09/160,976 entitled “Hermetically SealedUltra High Temperature Silicon Carbide Pressure Transducers and Methodfor Fabricating Same” filed Sep. 25, 1998. It also makes possible highertemperature application of the device since the contact material in theapertures is sealed from any high temperatures, hostile environmentwhile still retaining all of the advantages of the structure disclosedin pending U.S. patent application Ser. No. 09/160,976 entitled“Hermetically Sealed Ultra High Temperature Silicon Carbide PressureTransducers and Method for Fabricating Same” filed Sep. 25, 1998.Additionally, the use of a separate mounting surface for the sensorstructure makes possible the use of a header specifically designed forhigh pressure while still employing a miniature sensor.

Having described the preferred embodiment of this invention, it isevident that other embodiments incorporating these concepts may be used.Accordingly, although the invention has been described and pictured in apreferred form with a certain degree of particularity, it is understoodthat the present disclosure of the preferred form has been made only byway of example and that numerous changes in the detail of constructionin combination and arrangement of parts may be made without departingfrom the spirit and scope of the invention as here and after claimed. Itis intended that the patent shall cover by suitable expression in theappended claims, the whatever features of patentable novelty exist inthe invention disclosed.

What is claimed is:
 1. A pressure transducer comprising: a siliconsubstrate including a first surface adapted for receiving a pressureapplied thereto, an oppositely disposed second surface, and a flexingportion adapted to deflect when pressure is applied to said firstsurface; at least a first sensor formed on said second surface andadjacent to a center of said flexing portion, and adapted to measuresaid pressure applied to said first surface; at least a second sensorformed on said second surface and adjacent to a periphery of saidflexing portion, and adapted to measure said pressure applied to saidfirst surface; and, a glass substrate secured to said second surface ofsaid silicon substrate.
 2. The transducer of claim 1, further comprisingat least a third sensor formed on said second surface and adjacent tosaid center of said flexing portion, and adapted to measure saidpressure applied to said first surface.
 3. The transducer of claim 2,further comprising at least a fourth sensor formed on said secondsurface and adjacent to said periphery of said flexing portion, andadapted to measure said pressure applied to said first surface.
 4. Thetransducer of claim 3, wherein said first and third sensors are furtheradapted such that resistances respectively associated therewith decreasewhen said pressure to be measured is applied to said first surface ofsaid silicon substrate.
 5. The transducer of claim 4, wherein saidsecond and fourth sensors are further adapted such that a resistanceassociated therewith increases when said pressure to be measured isapplied to said second surface of said silicon substrate.
 6. Thetransducer of claim 5, wherein said first, second, third and fourthsensors are formed within said flexing portion.
 7. A pressure transducercomprising: a silicon substrate including a first surface adapted forreceiving a pressure applied thereto, an oppositely disposed secondsurface, and a flexing portion adapted to deflect when pressure isapplied to said first surface; at least a first sensor formed on saidsecond surface and adjacent to a center of said flexing portion, andadapted to measure said pressure applied to said first surface; at leasta second sensor formed on said second surface and adjacent to aperiphery of said flexing portion, and adapted to measure said pressureapplied to said first surface; a glass substrate secured to said secondsurface of said silicon substrate; a plurality of contact areas formedon said second surface of said silicon substrate; a plurality ofapertures in said glass substrate, each corresponding to one of saidplurality of contact areas; a plurality of conductive frits, eachpositioned within a corresponding one of said plurality of apertures;and, a plurality of conductive balls each also positioned within acorresponding one of said plurality of apertures.
 8. The transducer ofclaim 7, further comprising: a header including a plurality of pins;and, a mounting board coupled between said header and said glasssubstrate and adapted to respectively electrically couple each of saidballs to a corresponding one of said pins.
 9. The transducer of claim 8,further comprising a plurality of apertures in said mounting board, eachof said apertures in said mounting board corresponding to either one ofsaid plurality of pins, or one of said plurality of balls.
 10. Thetransducer of claim 9, wherein said mounting board comprises a materialselected from the group consisting of: a polymide and a ceramic.